Ferromagnetic-like behavior of Pt nanoparticles
Antipov S.D.1, Gorunov G.E.1, Perov N.S.1, Pivkina M.N.1,
Said-Galiyev E.E.2, Semisalova A.S.1, Stetsenko P.N. 1
1
Lomonosov MSU, Faculty of Physics, Moscow, GSP-1, 119991, Leninskie Gory, 1, bld. 2, Russia
2
INEOS RAS, Moscow, GSP-1, 119991, V-334, Vavilova Str., 28, Russia
gge@plms.ru, mpmsu@mail.ru
Keywords: magnetism, nanoparticles, ensemble of clusters, chemical method, ferromagnetic-like
behavior, Stoner criterion.
Abstract. The magnetic properties of small 4d, 5d metal nanoparticles of Pd, Pt (clusters) are
attracting a great attention because these materials in bulk are paramagnetic. In this work we report
the ferromagnetic-like behavior of the small Pt nanoparticles prepared by chemical method. Highly
dispersed Pt clusters have been synthesized on the surfaces of a porous spherical γ-Al2O3 particles.
The process of the chemical deposition of metalorganic fluid with employment of the supercritical
fluid was used. The samples of the Pt/γ-Al2O3 nanoparticles have been prepared in INEOS RAS.
The nanoparticles size distribution was determined by small-angle X-rays scattering (SAXS). It was
found that the Pt clusters have a bimodal particle size distribution with two peaks: R1max=20 Å and
R2max=40 Å. The magnetic properties of the clusters have been investigated, using VSM
magnetometer, in magnetic field up to ±3 kOe and at a temperature range from 80 to 400 K. It was
observed that Pt/γ-Al2O3 nanoparticles show the ferromagnetic-like behavior in whole specified
temperature range, the value of coercivity decreases gradually from 130 Oe to 80 Oe. The origin of
ferromagnetic-like behavior of the Pt/γ-Al2O3 nanoparticles is discussed.
Introduction
The properties of the nanostructured metallic clusters, supported on metal oxide substrates, are of
a great interest in the field of nanoscience and nanotechnology. This interest is primarily caused by
the importance of small metal clusters in heterogeneous catalysis and the fact that the physical
properties of these nanoparticles differ crucially from that of the corresponding bulk metals [1,2].
The origin of these new physical properties is the reduction of the nanoparticles size which, in turn,
causes (1) the changes in electronic states of the cluster atoms and (2) an increase in the ratio of the
number of surface atoms to inner ones (as well known, namely surface atoms form different bonds
with other atoms and ions). The ferromagnetic-like behavior was observed for the nanoparticles of
paramagnetic transition metals (Pd and Pt, in bulk) and diamagnetic Au (in bulk), when they were
capped with certain organic molecules [3-6]. In present paper we discuss the appearance of
ferromagnetic-like behavior for Pt nanoparticles prepared by a chemical method.
Experiments
The metallic Pt nanoparticles, supported on the surfaces of porous spherical γ-aluminium
particles (γ-Al2O3), were synthesized by the method of supercritical fluids deposition [7] using the
original experimental setup in INEOS RAS. The certain amount of organometallic precursor and
γ-Al2O3 powder were placed into the vessel of the experimental setup and then the system was
pressured with carbon dioxide (CO2) up to the necessary pressure. The particles of Pt/γ-Al2O3
nanocomposites were prepared using the platinum precursor (dimethyl (1,5-cyclooctuduene)
platinum (II) CODPt(CH3)2), dissolved in the carbon dioxide (CO2) as a supercritical fluid (sc CO2),
at 20MPa of pressure and a temperature of 100oC during 7 hours. When the vessel was cooled and
depressured, the impregnated precursor was reduced thermally (at 200 oC in the presence of argon
gas with a flow rate of 20cm3min-1 during 4 hours). As it was shown in [7], the reduction
temperature is a significant factor affecting the platinum particles size and the rise of the reduction
temperature of CODPt (CH3)2 from 200 oC to 600 oC results in an increase of the particles size from
23Å to 36Å. Therefore, in order to obtain small atomic aggregates (clusters) with high catalytic
activity, the temperature of CODPt (CH3)2 reduction was chosen as 200 oC. The commercially
available γ -Al2O3 (Katalizator IKT-02-6M) was used without further purification. The phase
composition of Pt/γ-Al2O3 particles was determined by XRD with Cu Kα radiation. The diffraction
curve indicated the presence of high amount of γ-Al2O3 phase (cubic, a = 7,9 Å). In 2Θ-region (3556)o strong diffraction peaks of γ-Al2O3 particles were observed, the peaks from Pt nanoparticles
were practically absent. The amount of Pt on the surfaces of γ-Al2O3 porous spherical particles was
determined by the analysis of Pt line (Lα), which was obtained using the spectrometer of
conventional X-ray fluorescence analysis VRA30 (Germany). The weight content of Pt in γ-Al2O3,
determined by this method, was 4,3%, i.e. the prepared supported nanoparticles were 4,3 wt%
Pt/γ-Al2O3. The pore size distribution of γ-Al2O3 particles and the size distribution of Pt
nanoparticles were evaluated by the method of the small-angle X-ray scattering (SAXS) using
“AMUR-K” X-ray diffractometer with Cu Kα radiation. The volume distribution function,
Dv(R)=N(R)  V(R) (where N(R) is the number of Pt nanoparticles of the similar spherical shape with
radius R and V(R) =4/3  πR3 is the volume of the particles with radius R), was obtained from the
analysis of the data of SAXS for Pt nanoparticles.
Figure 1 demonstrates the dependence of Dv(R) on
Pt nanoparticle size distribution. This result shows that
the Pt cluster ensembles have a bimodal particle size
distribution with two peaks: R1max=20 Å and
R2max=40 Å; analysis of SAXS data showed no any
evidence of particles agglomeration. It was found that
the particles of γ-Al2O3 have a roughly spherical shape
(like a pellet) with a diameter of ~ 0,7 mm.
Magnetic measurements were carried out using
“Lake Shore” vibrating sample magnetometer (VSM).
The
basic
magnetic
parameters:
saturation
Fig. 1 Dependence of the volume distribution
function, Dv(R)=N(R)  V(R), of Pt nanoparticles
magnetization (Is), remanent magnetization (Ir) and
as a function of their radius, R.
coercivity (Hc) were determined on the basis of the
hysteresis loops. The temperature dependence of
magnetization of Pt/γ-Al2O3 nanoparticles under magnetic field of 3 kOe was measured within the
temperature range 80-400 K.
The diamagnetic contribution of γ-Al2O3 particles and sampleholders were taken into account.
Experimental results and discussion
The magnetic properties of the Pt/γ-Al2O3 nanoparticles were measured in magnetic fields up to
±3 kOe and at a temperature range from 80 to 400 K. Figure 2a shows the hysteresis loops of the
magnetization measured in the temperature range between 80 and 170 K in magnetic fields up to
±3 kOe. As it follows from these magnetization curves, the Pt/γ-Al2O3 nanoparticles have a
saturation of magnetization in magnetic fields higher than 1 kOe and at temperatures up to 400 K;
and even at higher temperatures (above 400 K) they still have hysteresis properties and coercivity.
Accordingly to these results it can be concluded that (1) the nanoparticles of Pt/γ-Al2O3 exhibit a
ferromagnetic-like behavior in the whole temperature range 80-400 K and (2) they exhibit a
remanent magnetic moment. Measurements of magnetization of the sampleholder and “pellets” of
γ-Al2O3 have revealed their diamagnetic behavior just. After subtracting the diamagnetic signals we
have only the contribution from magnetization of Pt nanoparticles. This result indicates that the
ferromagnetic-like behavior of magnetization origins from the Pt nanoparticles located on the pores
20
Magnetization [memu/g]
Magnetization [memu/g]
80 K
130 K
170 K
350 K
400 K
40
0
-20
-40
-3000
-2000
-1000
0
1000
2000
3000
35
30
3 kOe, 80 - 350 K
3 kOe, 400 - 80 K
IS at M(H) curves
50
100
Magnetic field [Oe]
150
200
250
300
350
400
450
Temperature [K]
a)
b)
Fig. 2 (a) the hysteresis loops of magnetization of Pt/γ-Al2O3 nanoparticles at different temperatures (80-400 K); (b) the
temperature dependences of magnetization of Pt/γ-Al2O3 nanoparticles in a magnetic field of 3 kOe and the values of the
saturation magnetization IS derived from the hysteresis loops at various temperatures.
Coercivity [Oe]
surfaces of spherical γ-Al2O3 particles. To study the magnetic states of Pt/γ-Al2O3 nanoparticles, the
temperature dependences of magnetization I(T) in a magnetic field of 3 kOe were measured. Firstly,
the Pt/ γ-Al2O3 sample was cooled down to 80 K in zero magnetic field; the data of magnetization
were taken at 3 kOe magnetic field under slow warming up to 400 K and followed cooling back to
80 K. The results of these measurements are presented in Figure 2b, where the magnetization I of
the Pt/γ-Al2O3 nanoparticles is shown as a function of temperature T. We can see there two peaks,
which, probably, reveal two magnetic phases in Pt/γ-Al2O3 nanoparticles. That might be
corresponded to the bimodal particle size distribution with <R1> (mean radius)  20 Å and
<R2>  40 Å with different magnetic anisotropy constants, <K1> and <K2>, correspondingly.
In addition, we observed a high coercivity of
140
Pt/γ-Al2O3 nanoparticles in the whole studied range of
130
temperatures. Accordingly to the data presented in
120
Figure 3 the value of coercivity decreases with the
increasing temperature from 130 Oe at 80 K to 80 Oe
110
at 400 K.
100
All these experimental results indicate that the
90
nanoparticles of Pt/γ-Al2O3 synthesized by the
80
chemical method described above exhibit the
50
100
150
200
250
300
350
400
450
ferromagnetic-like behavior in the whole range of
Temperature [K]
temperatures (80↔400) K.
In order to understand the nature of the Fig. 3 The coercivity of Pt/γ-Al2O3 nanoparticles
ferromagnetic-like
behavior
of
Pt/γ-Al2O3 as a function of temperature.
nanoparticles and the very high calculated values of
magnetic anisotropy it should be noted that the catalytic activity of platinum on γ-Al2O3 particles are
caused by the interactions of Pt nanoparticles with their oxide support (γ-Al2O3) and the charge
transfer between the atoms of Pt clusters and γ-Al2O3 [8,9]. The atoms of Pt on the surfaces of its
clusters have a low coordination and narrow width of electron density of states at the Fermi level.
That leads to an increase in spin moment, S , and a greater degree of the orbital moment, L, of the
atoms of Pt clusters, in comparison to those for the bulk atoms of Pt. The charge transfer and the
formation of the ensembles of Pt clusters result in the high density states at the Fermi level: NPt(EF)
> NPt(EF bulk) = 0,9 eV-1. The interatomic exchange integral (Stoner parameter), IPt, ranges between
0,59 eV and 1 eV. We can see that the Stoner criterion for ferromagnetism: NPt(EF)  IPt ≥ 1 may be
satisfied for Pt/γ-Al2O3 nanoparticles.
The huge values of magnetic anisotropy and coercivity are attributed, most probably, to the large
value of spin-orbital coupling,
Hso=ξ(r) · L · S,
where L is orbital momentum and S is a spin one of Pt atoms of cluster and ξ(r) ~ 1/r · δV/δr is a
constant of spin-orbital interaction (V is a potential on the site of Pt atoms). As it seen, the value of
ξ(r) is dependent on the charge of surrounding atoms, coordination and geometry of supported
clusters. Thus, the value of spin-orbital coupling of Pt atoms of the clusters on the surface of γAl2O3 particles can be higher then those for the atoms of bulk Pt. Moreover, the value of HS0 will be
increased with the increasing orbital and spin momentums of Pt atoms of the clusters.
Conclusions
The small Pt nanoparticles (the ensembles of Pt atom clusters) were synthesized by a chemical
method. The process of the chemical deposition of metalorganic fluid with employment of the
supercritical fluid was used. Highly dispersed Pt clusters were synthesized on the surfaces of the
porous spherical γ-Al2O3 particles. The ferromagnetic-like behavior was observed in the temperature
range 80-400 K, the values of coercivity decrease slowly from 130 Oe to 80 Oe. The observed
ferromagnetic-like behavior was discussed in the terms of the electronic bond mechanism (Stoner
criterion). The huge values of magnetic anisotropy and coercivity can be explained by the
substantial values of spin-orbital coupling in the supported Pt/γ-Al2O3 nanoparticles.
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